Your generator cannot generate a password with the letters a, b or c due to the +3 when selecting the random letter. See rand, which already returns a number in the range from 0..$len.

The variable $len is named wrong - it has the last index of @lett, not the number of elements in the array. This means that your password generator would never output the last element from @lett if it weren't for the + 3 from the previous step. I would use:

Other than that, using double quotes in a simple assignment is weird unless you have a special reason to do so - and if you are starting with Perl just now, those special reasons ("stringification") are still far away. So, no assignment should be of the form